The Conservatives gather for their conference in Manchester this week knowing that the next election is for them to lose. Nothing that happened at Labour's gathering in Brighton changed the settled conviction among most of the government's senior figures that they are headed for defeat.

By choosing yesterday to announce that Gordon Brown will take part in leaders' television debates, Labour was clearly hoping to do a spoiler on the Tory conference build-up. Yet that was also a further sign of Number 10's desperation about the government's unpopularity. No previous prime minister has conceded to a debate because no previous prime minister has wanted to give equality of status to his opponents. Mr Brown has agreed to head-to-head combat with the other leaders only because he has nothing left to lose. Peter Mandelson, in his extraordinary conference vaudeville act, told his party that if he could come back, they could all come back. Members of the cabinet would regard it as a very successful comeback to keep the size of the Tory majority after the next election down to low double figures.

After more than a dozen years in opposition, Conservative nostrils are quivering with the smell of red boxes and limousine leather. Power is so close that they can almost taste it. One temptation for the Tories will be to succumb to premature triumphalism. David Cameron will be wise to stamp hard on any of that at Manchester. Voters will rightly recoil if they detect a revival of the arrogance and sense of entitlement that they grew to hate about the Tories when they were last in office. As one member of the shadow cabinet says: "We must not look like we are taking the electorate for granted." Mr Cameron has issued a script about this to his frontbench and they are all dutifully parroting the line that it will be a "workmanlike" conference at which the country will be shown a Conservative party soberly preparing for government.

This is about tone. It is also about maths. Labour losing the election is not the same thing at all as the Conservatives winning it. Only a very modest swing to the Tories is needed to deprive Labour of office. A much bigger swing is required to place power securely in the hands of the Conservatives. To win a parliamentary majority of one, the Tories must gain 117 seats. A shift of that magnitude has not been achieved since the very extraordinary circumstances of the election of 1931.

Another reason for Tories to eschew any triumphalism is the palpable lack of public enthusiasm for a Conservative government. The Tory poll rating bobs around 40 points. That is not at all impressive against an old and tired government led by a poor communicator who makes serial blunders and is presiding over a severe recession.

The Sun is not as important as it likes to think it is, but it is a useful weathercock. The tabloid's switch of allegiance back to the Tories was not interesting because it happened. That has been on the cards for a long time. What was more telling about the political climate was the manner in which the Sun executed its switch. When the red-top went Labour before the 1997 election, it did so by splashing a picture of a beaming Tony Blair on its front page and blaring a fanfare of adulation for him as a leader. They did not do the same for David Cameron. "Labour's lost it" was their headline, not: "The Tories have got it". The Tories themselves know that their poll rating is more a function of the rejection of Labour than an enthusiastic embrace of themselves.

Their greatest electoral asset is David Cameron. Tory strategists say they always gain whenever their leader is seen a lot on television, which is why they will try to get him wall-to-wall coverage at the conference and are sounding pleased about the prospect of TV debates.

The Tory leader's personal ratings are higher than those of his party. The plus is that he is dragging his party up behind him. The minus is the implication that his clever marketing has still not detoxified the Tory brand even among voters willing to give them a go.

The Conservatives remain vulnerable to the suspicion that there is something phoney and skin-deep about their reinvention. Both Tory and Labour pollsters tell me the same thing about their focus groups. A lot of voters still mention that unfortunate incident of some time ago when it was revealed that the biking Cameron was followed into work by his chauffeured limousine carrying his papers and suit.

The electorate remain sceptical of the authenticity of the Tories and uncertain about what they would get from a Conservative government. That is not surprising when many of their messages are contradictory. One of the more attractive, and politically winning, features of David Cameron's leadership has been to reconcile his party to the social liberalism of the New Labour years. At past conferences, he has even managed to persuade his party to applaud gay marriages.

The early phase of his leadership was to position the Tories as a softer, more likeable, more centrist party. Yet they are welcoming to Manchester some of their strange, new, far-right friends in Europe who include alleged antisemites and admirers of the Waffen SS.

The biggest disjunction between soft Cameroonian crooning about being progressive and the cutting edges of Tory policy is in the area of tax and spend. David Cameron began his leadership by projecting himself as an updated version of the 1950s One Nation Tory, Harold Macmillan in a T-shirt and wearing Converse. His rhetoric was designed to be unifying and uplifting. "Let sunshine rule the day!" he cheerily told his first party conference as leader three years ago.

The financial crisis triggered the abrupt change in approach that has sent the Tories back to being flinty fiscal conservatives. Now Mr Cameron repeats the homilies of Margaret Thatcher about good housekeeping and grimly warns us to be braced for "an age of austerity". The Tories are conscious of the risk of reviving memories that many people were left to rot on dole queues during and after the savage recession that she presided over in the 1980s. Hence the emphasis they are placing today and tomorrow on their proposals to address unemployment. That does not resolve the tension between their desire to be seen as "compassionate Conservatives" and the implications of their plan to implement cuts to public spending far deeper than anything done by Mrs Thatcher. For the last year or so, David Cameron and George Osborne have sought to make the size of the deficit the defining economic issue. They first junked their previous commitment to match Labour spending levels, they opposed the VAT cut and then they hammered their sound-bite "the cupboard is bare".

They have had a tactical success with that, aided and abetted by Gordon Brown's stubborn and self-defeating refusal to move to a more defensible position. The deficit is now the defining economic issue for many voters and much of the media. Having made it so, the pressure is now on the Conservatives to start being honest about where the axe would swing. With masochistic bravado, members of the shadow cabinet talk privately about being the most unpopular government of the past 50 years within months of taking office. They will certainly become so if they take power without being plain about what they intend to do. As the axe swings on services valued by the public, the Tories take the great risk that the voters will turn round and say: "We didn't think we were voting for that." George Osborne will tomorrow respond to this pressure by being a bit more specific about where he would cut. The shadow chancellor has warned his colleagues that their lives won't be worth living if voters think they got to government on a fraudulent mandate.

Scrapping ID cards and taking a scythe to quangoland are easy, populist cuts that will have a trivial impact compared with the scale of government borrowing. Had Gordon Brown proposed, as David Cameron recently did, an increase in the cost of a pint of beer in the bars of the Commons, he would have been rightly ridiculed.

The Tory leader only got away with that because the Conservatives are the least scrutinised opposition party in memory. Far too little attention has been paid to what their policies actually are and whether they would work. Labour has been too feeble and obsessed with its own difficulties to put serious heat on their opponents. The media has found it much more fun – and much easier too – to taunt Gordon Brown for his failings and trace the trajectory of Labour's decline.

This absence of interrogation is only superficially to the advantage of the Conservatives. One consequence is that there is no solid buy-in to the Tories from voters who are still not clear what voting Conservative will mean beyond being a means of booting out a deeply unpopular government. Another result of this lack of scrutiny is that Tory proposals are not being put in the pressure cooker to test whether they are a viable plan for power.

In Manchester this week, the Conservatives will invite everyone to treat them as the next government. That is exactly what everyone should do.